This invention relates to shielded electric cable, especially cable designed for high voltage service, notably service requirements at about 2000 volts and above, of which particularly important examples are in the range of approximately 5000 volts and upwards. The present cable is capable of use in electrical power circuits operating indoors, aerially, or particularly underground. In a more specific sense, the invention is concerned with cable of the type that includes a central power conductor, whether a single metal element or most usually a multiplicity of metal strands or wires twisted in a unified configuration, having a solid, space-filling layer of semiconducting material around it and tightly surrounded by insulation, e.g., a thick insulating body of polymeric material (for instance, extruded or molded in place), which is in turn closely jacketed or coated with semiconducting material for protective shielding function.
In cables of this type, it has been found unusually important to protect the insulation from the effects of electrical discharge, for instance as might occur in the nature of corona discharge in even small air gaps adjacent to the outermost surface of the electrical conductor element, whether of the single or multiple type, or in the immediate vicinity of localities around the exterior of the insulation. This protection is especially important when the insulation consists of one of various polymers now employed for such purposes, for example polyethylene, polypropylene, or the like, because discharges of this type, involving intensely ionized paths, can be seriously damaging to the insulation, by reason of chemical breakdown, gas release, or other fracture or fissure in the insulating structure, due to the heat and impact of the discharge. The consequence of such damage to the heavy, solid, insulating layer is the hazard that the insulation will fail in its function, causing breakdown of the power line.
To inhibit occurrence of these effects, especially in the case of the monolithic polymer bodies of insulation, the above coatings of semiconducting material, of plastic composition solidified in place, have been applied to the outer surface of such insulation, and also to the outside of the electrical conductor unit, covering it and filling the outer interstices between wire strands, before it is coated with the chief insulation. The function of the semiconducting materials, which are composed of a similar polymeric base in which is dispersed a suitably large quantity of conducting substance such as carbon, is principally to substitute such material as a non-ionizing leakage path instead of air everywhere around the inside and outside of the insulation, as well as to reduce the effective electric field or to make it more uniform, along such surfaces. Thus the semiconducting layer can be considered to provide a non-ionizing, low resistance, leakage path, uniformly throughout these regions, so as to diminish or obviate the possibility of corona or like discharges.
As an important addition to structures of the foregoing sort, it has been the practice to apply an outer metallic assembly, such as a wrapping of one or more bare wires of copper or tinned copper spaced around the insulated and shielded cable, which in turn serves a further purpose for the protection of the cable and avoidance of undesired breakdown. This metallic structure can constitute further electrical shielding by affording a surrounding system of elements that exhibit a neutral or ground potential. It can also serve as a so-called drain conductor, e.g., for carrying any transient or sudden flow of current caused by unusual electrical disturbance which might otherwise produce complete breakdown of the insulation and lead to short circuit of the main conductor.
In some instances, it has been proposed that the outermost semiconducting layer have imbedded in it thin metallic ribbons or fine wires or the like, which may extend lengthwise of the cable, either in a wrapped or longitudinally parallel disposition, to constitute a metallic shield for purposes somewhat similar to those just described. While these relatively light aluminum or copper elements or similar structures which are thus encased in or covered by the outer semiconducting layer can serve, so to speak, as a ground plane, they may not be entirely adequate for draining a high fault current if such occurs.
With the previously mentioned outer wrapping of spaced, bare wires, the drain for fault current can be adequate, and indeed the shield wires may even collectively serve as grounded-side, return service path for the electrical power circuit of which the central conductor is at high potential above ground. A single such cable assembly can thus provide single phase service. For three phase power transmission, using a group of three such assemblies, the outer wires can together constitute the common or neutral return for the system.
Another prior proposal has been to utilize a single large conductor coated with semiconducting material and wrapped around a main insulated power conductor of the same basic type as above, i.e., having an insulating layer provided with layers of semiconducting material under and over it. In such structure, the single, coated, outer conductor, which may be copper or aluminum, can serve effectively for the ground return of the power circuit, and it accommodates a high fault current effectively when such occurs, but it does not provide any uniformity or indeed efficacy of grounded or neutral structure for electrical shielding purposes. The single external conductor, moreover, affords relatively little mechanical protection in underground use; for instance during unguided digging in the vicinity of the buried cable, a digging implement could likely strike the insulated power conductor itself, with corresponding likelihood of direct damage to it.
As will be inferred from the foregoing, the present invention overcomes deficiencies of prior types of shielded cables such as have been mentioned above, particularly in respect to the nature and cooperative function of the outermost shielding structure. Furthermore, an important purpose of the invention is to achieve a simple, relatively less expensive cable design which affords superior results, especially in its protective features.